Islamic law considers as a major sin any act that the religion has warned Muslims against performing, or has promised harsh punishment for committing, or has prescribed a hadd for it.
Examples of major sins are disbelieving in Allah (God in Islam) after having believed in Him, killing an innocent soul, dealing with riba (usury, interest), treating one's parents harshly, adultery, fornication, giving false testimonies, etc.
"When fahisha (sexual immorality/obscenity/adultery) spreads openly in a nation, there is an outbreak of disease in the form of plague epidemics.
Indeed, salah prevents one from indecency (fahsha) and evil deeds (munkar).
Allah knows what you do (Surah Ankabut: Verse 45) This article about Islamic ethics is a stub.